1. Field of the Invention
The present invention relates to an optical sheet and an optical part made of transparent resin. More particularly, the present invention relates to an optical sheet and an optical part made of transparent resin obtained by subjecting a transparent resin to a specific heat treatment. The optical part of the present invention can be prepared by an efficient method. The product thus obtained has a light weight and excellent heat resistance, light resistance and impact resistance. The product is also advantageous in that if used in a planar form, it has a reduced thickness.
2. Description of the Related Art
An inorganic glass has heretofore predominated in the materials to be used in the preparation of products in the optical art. This is attributed to the fact that an inorganic glass inherently has an excellent light transmittance over a wide wavelength range, excellent heat resistance, chemical resistance, water resistance, surface hardness and abrasion resistance and a wide range of linear expansion coefficient.
However, the foregoing-inorganic glass is disadvantageous in that it requires a polishing step and much time to be finely worked on the surface thereof, if necessary. Thus, the foregoing inorganic glass cannot be fairly mass-produced and cannot be efficiently produced in a small amount for each of many varieties.
Further, the foregoing inorganic glass is required of low birefringence index if used for some glass parts. However, the foregoing inorganic glass cannot always attain birefringence as desired.
Moreover, an attempt has been made to apply resins such as polymethyl acrylate, polycarbonate, polyarylate and cyclic polyolefin to the art of optical parts taking advantage of its characteristics and transparency. An example of this attempt is to develop the application of such a resin to optical disc by reducing its optical distortion.
As such an example there has been noted the development of the application of these resins to the art of parts requiring a low birefringence index such as liquid crystal display board and liquid crystal projection optical parts. In these uses, these resins are required of birefringence index of not more than 15 nm, preferably not more than 10 nm, more preferably not more than 5 nm, most preferably not more than 2 nm, as calculated in terms of retardation value in single pass. These resins are required of good retention of shape at a temperature of 100.degree. C. to meet requirements in various processings or practical use.
In an attempt to solve the foregoing problems, JP-A-60-222241 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a process for solvent-casting a heat-resistant resin such as polyether sulfone. However, this process can difficultly allow the solvent to evaporate while maintaining desired surface precision on an industrial basis if the product has a thickness excess 100 .mu.m. Thus, a sheet or molded product having a thickness of greater than 100 .mu.m suitable as liquid crystal display board or optical member cannot be obtained from the standpoint of self-supporting properties and gas barrier properties.
Contrary that, in the case of using a thermoplastically forming method such as a extruded-molding or injection molding, the optical distortion of the sheet or molded product grow large at a stage immediately after forming, and it is impossible to obtain the sheet or molded having the desired quality from the standpoint of optical properties.
JP-A-7-126375 proposes a method which comprises heating an extruded sheet laminated with a protective film at a temperature of not lower than its glass transition temperature under an ordinary pressure as a method for eliminating the optical distortion of a thermoplastically formed flat board.
However, the foregoing heat treatment at a temperature of not lower than the glass transition temperature of the extruded sheet can difficultly maintain the flatness of the flat board which has been provided during forming at a good reproducibility because the frozen strain developed at forming disappears with a dynamic change. As a result, the optical distortion of the extruded flat board cannot be eliminated, making it impossible to obtain a substrate having the desired quality from the standpoint of optical properties or precision in surface flatness, as described later in the comparative examples.